1. Field of the Invention
This invention relates to a non-contact eye pressure meter for pressurizing the cornea of an eye to be examined in a non-contact fashion to thereby detect predetermined deformation of the cornea and find the eye pressure value, and in particular to an apparatus in which the pressure applied to the eye to be examined is reduced to the necessary minimum, whereby the examinee's sense of discomfort is mitigated.
1. Related Background Art
Non-contact eye pressure meters in which fluid is emitted to the cornea of an eye to be examined and the predetermined deformation (planar surface) of the cornea by this fluid is detected to thereby find the eye pressure value are known, for example, from Japanese Patent Publication No. 38437/1979 and Japanese Laid-Open Utility Model Application No. 143402/1984. Describing the apparatus of Japanese Laid-Open Utility Model Application No. 143402/1984 as an example, this known apparatus, as shown in FIG. 6 of the accompanying drawings, is comprised of a cornea deformation detecting system in which a light emitted from a light source 1 passes through a lens 2 to provide a measuring light beam 3 which travels to the cornea E of an eye to be examined and the reflected light beam 4 reflected thereby is caused by a lens 5 to enter a light-receiving element 6, and an air pulse generating system in which a piston 8 is pushed by the rotation of a rotary solenoid 7 and the air in a cylinder 9 is compressed, whereby an air stream with its pressure variable with time is emitted from a nozzle 10 toward the cornea of the eye to be examined. There are further provided a pressure sensor 12 for measuring the pressure in the cylinder 9 and a processing circuit 11 connected to the light-receiving element 6 and the pressure sensor 12, and the cylinder internal pressure signal when the cornea deformation detecting system detects the predetermined deformed state of the cornea is measured and converted into the eye pressure of the eye to be examined. In the prior art apparatus of this type, the driving of the solenoid 7 is stopped usually after the predetermined deformation of the cornea is detected, and the load by excess air pulse is prevented from being applied to the eye to be examined. FIGS. 7( a) and 7(b) of the accompanying drawings show that state. The output x(t) of the light-receiving element 6 and the output Ps(t) of the pressure sensor 12 are both shown in FIG. 7(a), and the driving signal of the solenoid is shown in FIG. 7(b). The processing circuit 11 detects the peak of the output x(t) of the light-receiving element 6 at t=t.sub.0, detects that predetermined deformation has been created in the cornea of the eye to be examined, and reads a cylinder internal pressure signal P.sub.1, whereafter it renders a solenoid driving signal S(t) into a low level at t=t.sub.1 after a slight predetermined processing time .DELTA.t and stops the driving of the solenoid 7. Along therewith, the pressure in the cylinder 9 decreases and becomes as indicated by Ps(t) in FIG. 7(a). In FIG. 7(a), P's(t) shows the variation in the cylinder internal pressure when the driving of the solenoid is not stopped, and Ps(t) has a low peak as compared with P's(t) and the pressure of the air pulse emitted to the cornea of the eye to be examined is reduced.
However, in the above-described example of the prior art, the stoppage of the driving of the solenoid is effected after the detection of the predetermined deformation of the cornea and therefore, generation of excess air pulse cannot be completely eliminated and thus, the shock applied to the eye to be examined during measurement has not been sufficiently weak.
The eye pressure measurement by the apparatus of this type is usually effected three to five times and the representative value is adapted as the eye pressure of the eye to be examined, but the eye pressure value is much affected by the examinee's psychological condition. Accordingly, there has been the problem that if the shock of the air pulse during measurement is great, stable measurement cannot be accomplished in the second and subsequent measurements and a highly reliable measured value cannot sometimes be obtained.